Parallel line applications
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This type of networks are defined as those networks where the parallel transmission
lines terminate at common nodes at both ends.
The three most common operation modes are:
1. parallel line in service.
2.
parallel line out of service and grounded.
3. parallel line out of service and not grounded.
Parallel line in service
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This type of application is very common and applies to all normal sub-transmission
and transmission networks.
Let us analyze what happens when a fault occurs on the parallel line see figure
154.
From symmetrical components, we can derive the impedance Z at the relay point for
normal lines without mutual coupling according to equation 208.
ph ph
0 1 ph 0 N
ph 0
1
U U
Z Z I 3I K
I 3I
3 Z
Z = =
- + ×
+ ×
×
EQUATION2311 V1 EN-US (Equation 208)
ph ph
0 1 ph N
ph 0
1
0
V V
Z Z I 3 K
I 3I
3 Z
I
Z = =
- + ×
+ ×
×
EQUATION1275 V3 EN-US (Equation 208)
Where:
V
ph
is phase to ground voltage at the relay point
I
ph
is phase current in the faulty phase
3I
0
is ground fault current
Z
1
is positive sequence impedance
Z
0
is zero sequence impedance
Section 8 1MRK 504 163-UUS A
Impedance protection
330 Transformer protection RET670 2.2 ANSI
Application manual
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